It has already been proposed to utilize ozone as a bleaching agent for ligno-cellulosic materials of the type described in order to enable the bleaching to be effected as much as possible in a chlorine-free manner and thus with greater environmental protection.
Typical of such processes is a process in which the pulp suspension is treated with an ozone-containing gas under vigorous agitation. The term "vigorous agitation" is used herein to include vigorous mixing.
Indeed, chlorine-free and thus ecologically harmless bleaching of pulps, which can be worked up to paper or fibers, utilizing ozone, is described in numerous patents and publications. The various processes described differ primarily in the parameters of the process and the reaction conditions. An important parameter is the consistency, i.e. the percent by weight which is essentially equivalent to the mass percent of the solid pulp in the aqueous suspension.
In principle, these processes can be considered to be in either of two categories, namely, the high consistency (HC) or the low consistency (LC) techniques.
HC ozone bleaching is carried out with consistencies in excess of 25% and generally around 35 to 40%.
Since ozone bleaching normally has not been carried out as the exclusive bleaching operation but is generally provided in combination with other bleaching steps and conventional bleaching can scarcely be carried out at such high consistency ranges, expensive dewatering units must be provided to prepare the pulp suspension for the ozone bleaching. The reaction of ozone with pulp is a two phase reaction which proceeds rapidly to completion.
Aside from the high capital cost of equipment for carrying out HC bleaching for the reason given above, i.e. the cost of the dewatering units, a disadvantage of the HC process is a nonhomogeneous cellulose-damaging ozone attack which appears to be most pronounced at low initial kappa starting values.
(The significance and definition of kappa can be found in col. 2 of U.S. Pat. No. 4,229,252).
In the literature, therefore, it has been suggested that HC ozone bleaching should not be used at kappa values below 10 (Lindholm C. -A. "Effect of pulp consistency and pH in Ozonbleaching" Part 4 Paperi ja Puu--Paper and Timber 2/1989;
Lindholm C. -A. "Effect of pulp consistency and pH in Ozonbleaching" Part 2 1987 Int. Oxygen Delignification Conference, San Diego, Jun. 7-11, 1987, Proceedings, p. 155; Lindholm C. -A. "Effect of pulp consistency and pH in Ozonbleaching" Part 3 Nordic Pulp and Paper Research Journal, No. 1/1988.)
The cellulose damage is still worse when the cellulose is bleached prior to the HC ozone bleaching with oxygen.
The only alternative according to the state of the art is the LC ozone bleaching process if one wishes to avoid the use of chlorine containing environmentally hazardous compounds. The LC ozone bleaching process by comparison to the HC process utilizes more ozone, is more complicated to carry out and requires a greater amount of mixing energy. Furthermore, the reaction volumes are greater and the danger of importing dirt into the process is increased.
It is generally recognized in me art that LC refers to pulps with a consistency of up to 5 or 6%.
In the case of ozone bleaching, however, it is well recognized that only with a consistency of say up to 1% and at most 2% will usable results be obtainable.
For example, U.S. Pat. No. 4,216,054 emphasizes a consistency range of up to 0.7%. Such a consistency range means that the equipment must include a significant investment for a closed water circulation system. This patent describes systematic investigations of LC technology for kraft pulp and concludes that the reaction of ozone with the cellulose is limited by two barriers, namely, the transfer of the ozone from the gas phase to the liquid phase and the transfer from the liquid phase to the solid phase i.e. to the fibers from a minimum mixing power of 11 kW/m.sup.3 the second transfer remains rate determinative according to this patent.
An LC bleaching process is also described in U.S. Pat. No. 4,080,249. It is suggested that the agitation energy should amount here preferably to at most 18 kWh/t of the pulp suspension. The bubbles of the ozone containing gas should have a size of at most 3 millimeters. In all of the examples of this patent, consistencies of between 1 and 2% are described, thereby clearly indicating that the document refers to an LC process.
AS part of a broadcast disclosure, apparently to foreclose circumvention of the patent, mention is made of consistencies up to 10% although it is clear in any case that consistencies below 3% are preferred, thereby providing an equally clear indication that consistencies above 3% are not preferred or are detrimental.
Substantially the same can be said for U.S. Pat. No. 4,372,812. Here there is an equally broadcast disclosure of between 1 and 40% although the example only operates in the LC range, namely, with a consistency of 1% (see table 1 of this patent). This document also deals with a multistage bleaching process in which ozone is introduced into one or more stages but not with an ozone bleaching stage per se.